Question

pollution occurs when

harmful substances build to unhealthy levels.
there is too much of any substance.
a harmful substance is detected.
too many people handle the substances.

Explanation:

Pollution is defined as the presence of harmful substances in the environment at levels that are detrimental to living organisms or the ecosystem. It's not just about the presence of any substance in large amounts (not all substances are harmful), nor just about detection (low - level, non - harmful presence isn't pollution), and not related to the number of people handling substances. It's when harmful substances build to unhealthy levels.

Answer:

harmful substances build to unhealthy levels.